This invention is in the field of collimators for tube-launched weapon sights. A specific example of such a weapon is the U.S. Army TOW missile system. This system includes a launch tube from which a missile is fired and a sight affixed to and bore-sighted to the tube. The original sight of the system was intended for daylight use, but night sights using infrared detectors are now frequently used. Naturally, it is necessary to bore-sight the night sights with the launch tube. This may be easily accomplished by bore-sighting the night sight with respect to the day sight rather than directly to the launch tube. Unfortunately, there are currently three types of night sights: AN/TAS-4, AN/TAS-6, and GLLD, and each of these sights has a different mount from the others on the launch tube. It would thus seem that three different collimators would be required for these night sights, but a single collimator has been made capable of boresighting the three sights. This collimator, however, is not field-adjustable for its three sight modes, but must be set up in a maintenance depot having equipment to evacuate and purge the collimator after adjustment. Of course, the collimator is out of service for a field user during its trips to and from depot and while at depot. The instant invention provides a collimator which is readily adjustable in the field by its operator, and which requires no special training or equipment for such adjustment.